As the aerospace industry transitioned from hydrocarbon-based hydraulic fluids to phosphate-ester-based hydraulic fluids it became apparent that hydraulic components (e.g. valves) that had operated reliably using the hydrocarbon-based hydraulic fluids were degrading prematurely during operation. It is believed that the use of phosphate-ester-based fluids may contribute to the erosion of the valve components, leading to unacceptable valve performance (e.g. leaking valves). An analysis of the factors causing valve failure in hydraulic systems revealed that electrokinetically driven corrosion is a significant contributing factor. This corrosion is induced by the “sweeping of charge” in the electrical double layer that extends from the valve surface into the hydraulic fluid, resulting in electrochemical current driven between the valve and fluid. The term “sweeping of charge” is understood to mean putting a greater amount of charge in or out of fluid located at the valve surface. Replacement of the hydrocarbon-based hydraulic fluid with a phosphate-ester-based hydraulic fluid has therefore had the undesirable consequence of modifying the electrical double layer to produce larger electrochemical currents, and therefore more significant corrosion.
The magnitude of the electrokinetically driven current, for a given fluid velocity distribution, is primarily determined by the amount of charge in the electrical double layer, and the overlap of the charge distribution in the electrical double layer with the velocity profile in the hydraulic fluid. As new hydraulic system designs employ higher pressures and more advanced and more compact valve structures designed for more rapid operation, the resulting higher fluid velocities in smaller valve geometries produce substantially higher electrokinetically driven surface current densities.
Therefore, as valve geometries become more constrained and hydraulic operating pressures increase in hydraulic systems, the commercially available phosphate-ester-based hydraulic fluids, even with their additives, are incapable of reducing the electrokinetically driven surface currents.
The problem of hydraulic valve erosion has been addressed by including additives to the phosphate-ester-based hydraulic fluid that: (1) reduce the next surface charge, and hence the charge in the electrical double layer; and/or (2) increase the ionic conductivity of the fluid to collapse the electrical double layer so that the net ionic charge is localized much closer to the component surface, (i.e., in the region of much lower fluid velocity).